1. Field of the Invention
The present invention relates generally to power supplies and, more specifically, the present invention relates to a switching regulator.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today's electronic devices. In a known switching power supply, a low frequency (e.g. 50 or 60 Hz mains frequency), high voltage alternating current (AC) is converted to high frequency (e.g. 30 to 300 kHz) AC, using a switched mode power supply control circuit. This high frequency, high voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated DC output, which may be used to power an electronic device. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop.
The design of the switching power supply is a compromise among conflicting requirements of efficiency, size, weight, and cost. The optimal solution that delivers the rated output power usually sets the switching frequency much higher than 20 kHz, outside the range of human hearing.
Regulatory requirements call for power supplies to operate at high efficiency at low loads such as standby loads and consume very low power at no load. When a power supply delivers much less than its rated power, the energy lost within the power supply is dominated by losses from the action of switching. Therefore, it is necessary for the power supply to operate at lower switching frequencies when the output power is low to reduce the dominant losses. The switching frequency may be reduced linearly as the load reduces to maintain high efficiency. The optimal switching frequency at low power often falls within the band of audio frequencies below 20 kHz. Switching within the band of audio frequencies can produce undesirable audio noise in power supply components such as transformers and ceramic capacitors due to mechanical resonances.
A well-known technique to reduce switching losses and improve efficiency at light loads is to operate the power supply in a burst mode at light loads. Burst mode operation allows the power supply to switch an uncontrolled number of consecutive switching cycles at a high switching frequency followed by a duration of no switching adjusted in a closed loop to regulate the output. Thus, the average switching frequency is reduced to keep the efficiency high at light loads. An undesirable property of burst mode switching is that neither the number of consecutive high frequency switching cycles in the burst nor the number of consecutive cycles of no switching between bursts is determined for a given set of operating conditions. The indeterminate nature of burst mode operation creates the hazard of uncontrolled audio noise. In fact, if the repetition rate of the duration of consecutive switching cycles followed by the duration of no switching is at audio frequency, the audio noise could be worse than just reducing the frequency linearly as described earlier due to higher audio energy content. Audio noise is a major drawback of using burst mode operation for improving light load efficiency and reducing no load consumption.
One of the most troublesome sources of noise in a switching power supply is the transformer. Commonly used ferrite core transformers in switching power supplies tend to have mechanical resonant frequencies in the 8 kHz to 15 kHz range. Some ceramic capacitors, especially the ones used in the clamp circuit connected to the primary winding of the transformer in a flyback power supply, can also resonate at such audio frequencies.